The most successful and widely used of the pressure processes are those in which the wood treatment is carried on in closed cylinders. In general, such processes have a number of advantages over the nonpressure method. Of prime importance is the fact that, in most cases, a deeper and more uniform penetration and a higher absorption of preservative can be secured, thus providing more effective protection to the timber. Furthermore, the treating conditions may usually be so controlled that retention and penetration can be varied to meet the requirements of service, thus resulting in more economical use of preservative. Special manipulations, or preliminary treatments, within the cylinder also make it possible to impregnate unseasoned wood or to sterilize the timber. Finally, these pressure processes are adapted to the large-scale production of treated material.
While the various pressure processes differ in details, the general method of handling the material is the same in all cases. The treatment is carried on in cylinders or retorts, mostly within the limits of 6 to 9 ft. in diameter and up to 150 ft. or more in length, and built to withstand working pressures up to 250 psig. The ties, poles, structural timbers, or other forms of timber are loaded on special tram cars, which are moved about the treating yard and in and out of the cylinder on steel tracks. Storage and measuring tanks, pressure and vacuum pumps, steam boilers, and other plant equipment are also provided.
The various pressure methods used for injecting preservatives into wood in closed cylinders may be divided into two main groups, designated as full-cell and empty-cell processes. In the full-cell process, the aim is to retain as much of the liquid forced into the wood during the pressure period as possible, thus leaving the maximum concentration of preservative in the treated zone. In the empty-cell processes, on the other hand, part of the preservative forced into the timber under pressure is subsequently recovered, so that the cells tend to be coated with preservative rather than filled with it.
The full-cell process is especially advantageous when it is desired to inject as much preservative solution as the wood can take. Empty-cell methods are required when it is desired to secure as deep penetration as possible with a limited final retention of liquid. When wood is to be treated with preservative oils, the empty-cell processes are generally used.
When treating with water solution of preservative salts, it is customary to use the full-cell process and to force into the wood all of the liquid that it can hold. The desired retention (amount of dry salt per cubic foot of wood) is obtained by regulating the strength of the solution rather than by limiting the amount of liquid injected into the timber. The distinguishing characteristic of every full-cell treatment is the preliminary vacuum, the purpose of which is to exhaust the air from the cells of the wood.
As practiced today, full-cell treatment with water solutions embodies the same general procedure as with oils. The preliminary vacuum is followed by the admission of the preservative solution and the application of pressure, which may amount to as much as 200 psig. The solution temperatures range from atmospheric to 200.degree. F., depending on the preservative used. When virtually no more preservative can be forced into the wood, the pressure is released and the cylinder drained. The strength of the treating solution generally varies from 2 to 8 percent, depending upon the amount of liquid that can be forced into the timber; for preservative purposes it is usually so regulated as to give a retention of 1/4 to 21/2 lbs. of dry preservative per cubic foot of wood.
The chief characteristic of the empty-cell process is the application of preliminary air pressure to the wood, prior to the injection of the preservative oil. The timber to be treated should be air dry for best results, but green material may be used, provided it is first conditioned by the steaming-and-vacuum, the boiling-under-vacuum, or other suitable method. Air is injected into the treating cylinder until the desired pressure is obtained, with the result that a certain amount is forced into the wood. The cylinder is then filled with preservative in such a way that the injected air is trapped in the wood. The filling may be accomplished by forcing the preservative into the bottom of the retort and allowing air to escape from the top just rapidly enough to maintain a constant pressure. Another way of carrying out the operation is to have the preservative in an overhead Rueping tank under the same pressure as the compressed air in the cylinder and, by means of a suitable arrangement of pipes, to allow the oil to flow into the retort by gravity while the air passes up into the space vacated by the preservative. After the filling is completed, the preservative is forced into the timber by the application of higher pressure until the desired absorption is obtained, thus further compressing the air imprisoned in the wood. The pressure is then released, the preservative drained from the cylinder, and the charge subjected to a high final vacuum for a period of 30 minutes or more. As soon as the pressure is released, the compressed air in the wood expands and forces out a considerable amount of the preservative that was injected. The final vacuum hastens the recovery of oil and also shortens the period during which the preservative will continue to drip from the timber. The maximum amount of preservative contained in the wood at the end of the pressure period is called the gross absorption; the amount expelled after the release of the pressure but without the application of vacuum is termed the kickback; the amount recovered from the wood during the final vacuum is sometimes called the drip or drain; the kickback plus the drip constitute the total recovery; and the net amount left in the wood is known as the net absorption, or net retention. As the result of this recovery, the net retention of preservative associated with a given penetration is substantially less than would be required to saturate the wood with oil to the same depth. It follows that, with a limited net retention, deeper penetration is obtained in permeable woods by the empty-cell process than by the fuel-cell treatment.
The intensity of the preliminary air pressure is governed by the character of the wood being treated and the net retention of preservative desired. When low retentions are specified in wood that is very receptive to treatment, such as the air-dry sapwood of southern pine, air pressures as high as 100 psig may be used. When more refractory woods or higher net retentions are involved, lower pressures are employed; in any given case, the precise intensity will depend largely upon the judgment of the operator, but pressures of 25 to 75 psig are most common. In some plants, the practice is to start filling the cylinder with preservatives as soon as the air pressure is built up to the desired amount; while in others, the maximum pressure is maintained for 15 to 30 minutes before filling.
The pressures employed in injecting preservatives into the wood are commonly about 100 lbs. higher than the preliminary air pressures, but it is frequently inadvisable to increase them to such an extent. The preservative pressures seldom exceed 200 psig, the maximum permitted under the specifications of the American Wood-Preservers' Association, and it is usually unnecessary to raise them above 150 lbs. With woods of low compressive strength, or green timber that has been softened by steaming, it is often desirable to set the limit at 115 to 125 lbs., since greater pressures may tend to cause collapse and checking of such material.
The amount of preservative recovered from the wood upon release of preservative pressure and application of final vacuum varies widely. It is influenced by the character and condition of the wood, the relation between the preliminary-air and preservative pressures, the temperature of the preservative and, no doubt, various other factors. Recoveries as high as 50 to 60 percent of the gross absorption are sometimes obtained, but those of 20 to 40 percent are more common.
The Lowry process differs from the empty-cell process only in the fact that it does not involve the use of an initial air pressure above the atmospheric. The preservative oil is injected into the timber without any preliminary treatment other than such conditioning as may be required when green wood is to be impregnated. The air that is naturally present in the wood is compressed during the preservative-pressure period and serves to expel part of the injected oil when the pressure is released and the final vacuum is drawn. With oils the preservative temperature should average not less than 180.degree. F. and not exceed 220.degree. F., and the maximum pressure is limited to 200 psig (for some species it must not exceed 150 psig). The length of time required to attain the specified pressure and the period for which it is held vary according to the kind of wood being treated and other conditions, such as the size and moisture content of the material; in general, the maximum pressure being used is maintained until the desired gross absorption is obtained. The final vacuum is usually held for a period of not less than 30 minutes.
One of the problems found with treating wood with water-borne salts such as CCA salts is that the chromium, copper and arsenic precipitates out of solution when wood sugars enter the treating solution during the kickback or pressure release phase of the empty-cell cycle. For this reason the AWPA requires that a full-cell cycle be used with water-borne salts. The problem with using the full-cell cycle is that it adds 25 to 30 pounds of solution to the wood during the treating cycle, which requires that the treated wood be subjected to a prolonged period of seasoning to reduce the weight to a reasonable shipping level or the treated wood is shipped at an expensive high shipping weight.